13.给定两个向量a→=(3,4)\overrightarrow{a}=({3,4})a=(3,4),b→=(2,−1)\overrightarrow{b}=({2,-1})b=(2,−1),若(a→+xb→)⊥(a→−b→)({\overrightarrow{a}+x\overrightarrow{b}})\bot ({\overrightarrow{a}-\overrightarrow{b}})(a+xb)⊥(a−b),则xxx的值是((( )))